As one of substrates for semiconductor devices, there is an SOI substrate having a silicon layer (hereinafter sometimes referred to as an SOI layer) formed on a silicon oxide film, which is an insulator. This SOI substrate has characteristics, such as small parasitic capacitance, high radioactivity resistance, and the like, since the SOI layer in a substrate surface portion to be used as a device fabrication area is electrically separated by a buried oxide film layer (a BOX layer) from the inside of the substrate. For that reason, the SOI substrate is expected to provide effects such as high-speed and low-power consumption operation, soft error prevention, and the like, and thus it is considered as a promising substrate for high performance semiconductor devices.
As one of methods for producing this SOI substrate, a method called a bonding method is known. The method for producing an SOI substrate by the bonding method consists of three steps which are a step of initial bonding, a step of heat treatment, and a step of reducing film thickness (see, e.g., Japanese Unexamined Patent Application Publication (Kokai) No. 2003-37253, etc). As the method for the reducing film thickness, there are a method for, e.g., grinding or polishing a bond wafer until a desired thickness, a method for delaminating the bond wafer at an ion-implanted layer based on a method called an ion implantation delamination method, and others.
This method for producing an SOI substrate by the bonding method will be explained more specifically.
A flow chart showing an example of a conventional method for producing an SOI substrate by the bonding method is shown in FIG. 4.
First, as shown in FIG. 4(a′), a bond wafer (a single crystal silicon wafer to be an SOI layer for forming semiconductor devices) 61 and a base wafer (a single crystal silicon wafer to be a supporting substrate) 62 are prepared (step a′). Next, as shown in FIG. 4(b′), a silicon oxide film 63 to be a buried oxide film is formed on a surface of at least one of the single crystal silicon wafers (FIG. 4 shows an example in which the oxide film is formed on the base wafer) (step b′). Next, as shown in FIG. 4(c′), the bond wafer 61 and the base wafer 62 are closely contacted via the oxide film 63 and are bonded. In this way, a substrate 70 in which two wafers are bonded and which has a buried oxide film 73 and a bonded plane 64 is obtained (step c′).
Next, as shown in FIG. 4(d′), a heat treatment to increase bonding strength of the bonded two wafers is performed at a temperature of 1100° C. or more (step d′). After cooling, then, as shown in FIG. 4(e′), the bond wafer is thinned until a desired thickness by grinding, polishing, chemical treating, or the like, to be an SOI layer 71 (step e′).
In this way, by the bonding method, an SOI substrate 80 is produced.
However, if an SOI substrate in which a thickness of a buried oxide film is thick as 3 μm or more is produced by the conventional method as the above, slip dislocations (which are also just called slips) are generated and it has been impossible to obtain an SOI substrate having a high-quality SOI layer.